Media types are identified by the file extension. To post media in your podcast, a file extension is required. Files that are supported for post are called "publishable". Those that are not publishable are displayed in red in your media library.
PodOmatic supports many different audio/video media types for post. For posted episodes however, PodOmatic converts to the most widely accepted formats:

PodOmatic determines if a audio/video media file can be published using the following guidelines:
Does it have a file extension? If not, we cannot determine the file type.
Is the file over 350MB? If so, its too big to post.
Does the file have one of these extensions: .vob .ram .rm .aup .flv .cda .itl .wpl .m3u .pmf .mht .asf .pict .mswmm? If so, its not a supported format.
Can the file be opened, and its duration read? If not, we cannot convert to the proper format, because the file cannot be opened by our conversion software.
Note that if your file cannot be published, the reason why is given in the media library. Note also that non-audio/video files can be published, but they cannot be played and will require notifying your audience of what you're providing them.